1. Technical Field
The present invention relates to a technique of preventing display degradation when both phase expansion driving and video precharge are used.
2. Related Art
In recent years, projectors that form reduced images using an electro-optic panel such as a liquid crystal panel and project the reduced images on an enlarged scale through an optical system have been coming into widespread use. The projectors have no function of forming images by themselves, and receive image data (or image signals) from a host device such as a personal computer or a TV tuner. The image data designates the gray level (brightness) of each of pixels supplied in the form of pixel matrix scanned vertically and horizontally. It is therefore appropriate to accordingly drive the display panels for use in projectors. Accordingly, the display panels for projectors are generally driven by a dot sequential system in which scanning lines are selected in a predetermined order on a line-by-line basis, and in which data lines are selected one by one during the period of time that one scanning line is selected (a horizontal scanning period), and a data signal that is converted from image data so as to be suitable for driving liquid crystal is supplied to the selected data line.
High-definition display images, such as those shown in high-definition televisions, have been recently becoming more common place. The high-definition can be achieved by increasing the number of scanning lines and the number of columns of data lines. However, the frame frequency is fixed, so that one horizontal scanning period is decreased by an increase in the number of scanning lines. In addition, in the dot sequential system, the time suitable to select data lines is also reduced by an increase in the number of columns of data lines. Accordingly, the dot sequential system cannot provide sufficient time for supplying data signals to data lines to realize high definition image display with the advance of high definition, leading to insufficient writing to pixels. Thus, a phase expansion driving system was devised to solve the problem of insufficient writing (refer to JP-A-2000-112437).
This phase expansion driving system is a system in which data lines are divided into blocks every predetermined number of columns, for example, every six columns, and the blocks are selected in a predetermined order one by one in one horizontal scanning period, while data signals supplied via six image signal lines and extended to six times on time base are sampled and supplied to six columns of data lines in the selected block.
Since the data lines are formed close to each other on a substrate made of glass or quartz, parasitic capacitors are formed on the data lines. Accordingly, when data signals are supplied, the voltages of the data signals are held by the parasitic capacitors. Since the voltage of a data signal depends on display content, when a voltage according to the display content is sampled for a data line to write data into the line, the sampled voltage is held until writing to the next line. Accordingly, at the writing to the next line, the initial voltage immediately before the data signal is sampled for the data line might become different between the data lines.
In this case, even if the same voltage is sampled to have the same pixel gray level, the sampled voltage will become different because of the difference in initial voltage level. In order to prevent this from occurring, a technique of precharging all data lines with a predetermined voltage immediately before data signals are sent to data lines has been developed (refer to JP-A-10-171421).
Another technique of precharging is termed video precharge in which precharge signals of the same voltage are supplied to six image signal lines, and the precharge signals are sampled for all data lines to thereby precharge all the data lines.
However, in the technique of video precharge, although precharge signals of the same voltage are applied to all image signal lines, the voltages precharged to the data lines have slight differences, thus resulting in degradation in display quality due to the differences.